It's true that the speakers were not limited to being British, but it's more expensive for someone outside the country to attend (even with compensation, they'd still expend more time, undergo more stress, etc.). Also, it was a new conference, not a large and established one, so it seems natural that it would have mainly a local draw. They may have even made the choice to invite more local speakers, to promote the Manchester/UK tech scene, which would be a worthwhile goal for such a conference, although it's directly at odds with the diversity goal.
It's true that Ruby is a Japanese invention (I wonder if all these racists are aware of what they're bringing into their good ol' boys' club ;)), but as you note, Japan is so far away that there are still difficulties in participating in a little new conference in Manchester. Also, Ruby got its big break with Rails, which was developed in Chicago, I think, so the Japanese community might not have gotten as much of a headstart as one would expect. Or maybe they are using Ruby for far more interesting things than web development. I don't know.